The present invention relates to an optical encoder device, and more specifically to the mounting of an optical encoder device.
An encoder enables a signal interpretation such as to obtain information on a position, a velocity, acceleration and/or the like when the encoder works in pair with a codewheel or a codestrip. Codewheels are generally used for detecting the rotation motion, for example of the paper feeder drum in a printer or a copy machine, while codestrips are used for detecting the linear motion, for example of the print head of the printer.
Usually, the motion of the codewheel or the codestrip is detected optically by means of an optical emitter and an optical detector. Therefore, the encoder is usually an optical encoder. The optical emitter emits light in a light emission direction towards the codewheel/codestrip. The codewheel/codestrip includes a regular pattern of slots and bars. According to the position of the slots and bars, relative to the light emission direction, the codewheel/codestrip sometimes permits and sometimes prevents light passing through. The optical detector is positioned behind the codewheel/codestrip, when seen in the direction of the light emission by the optical emitter, and detects a light signal, based on the light emitted by the optical emitter and transmitted through the codewheel/codestrip. Said detected light signal is either quadrature and/or sinusoidal and the frequency of said light signal yields unambiguous information on the motion of the codewheel/codestrip.
Due to the special arrangement of the optical emitter and the optical detector of such an optical encoder, the optical encoder housing for accommodating the optical encoder is generally C-shaped. The optical encoder together with the C-shaped optical encoder housing form a C-shaped optical encoder device. The codewheel/codestrip is passed through the free space of the C-shaped optical encoder device and moves such that the optical encoder can detect the slots and bars inside the codewheel/codestrip. FIG. 2A and FIG. 2C show cross-sections and FIG. 2B and FIG. 2D show top views of such a C-shaped optical encoder device 201 together with a codewheel 202 or a codestrip 203, respectively. The codewheel 202 and the codestrip 203 are provided with a regular pattern of slots 204 which are arranged such that a motion of the codewheel 202 or the codestrip 203, respectively, is unambiguously detectable. Therefore, the codewheel 202 or the codestrip 203, respectively, is passed through the free space 205 of the C-shaped optical encoder device 201 which takes up the codewheel 202 or the codestrip 203, respectively. If the codewheel 202 is rotated around the center C in a direction indicated by the arrows 206, respectively if the codestrip 203 is linearly moved in a direction indicated by the arrows 207, the slots 204 cause an alternating light signal in the optical detector of the optical encoder which results in an unambiguous information on the motion of the codewheel 202 or the codestrip 203, respectively.
Generally, the C-shaped optical encoder device is mounted on a printed circuit board (PCB) which is positioned inside the appliance, e.g. a printer or a copy machine, and which is used for an electrical coupling of the optical encoder to the control unit of the appliance. However, to ensure an accurate signal feedback it is important to maintain an accurate, reliable and firm position of the C-shaped optical encoder device relative to the PCB with minimum mounting tolerance. To ensure an accurate position of the C-shaped optical encoder device relative to the PCB horizontal or rotational displacements of the C-shaped optical encoder device have to be avoided.
According to the prior art, positioning is achieved by means of auxiliary devices comprising a pair of rounded guideposts which are located underneath the C-shaped optical encoder device. FIG. 3A shows in a bottom view of the C-shaped optical encoder device 201 the pair of rounded guideposts 301. As an example the C-shaped optical encoder device 201 is shown in FIG. 3A with a codestrip 203, but the positioning is also suitable for a codewheel 202. The pair of rounded guideposts 301 is part of the C-shaped optical encoder device 201 and is formed on that sidewall of the C-shaped optical encoder device 201 which is in contact with the PCB. The PCB is provided with matching holes 302 for taking up the pair of rounded guideposts 301. FIG. 3B shows a side view of the C-shaped optical encoder device 201 together with the pair of rounded guideposts 301 as well as contact pins 303 for electrically coupling the optical encoder to the control circuit of the appliance. FIG. 3C shows a footprint of the matching holes 302 as well as of the contacting holes 304 for taking up the pair of rounded guideposts 301 as well as the contact pins 303 of the C-shaped optical encoder device 201. According to the described positioning system, there results a tolerance between matching parts with a maximum of typically 0.1 mm. However, this is not sufficient for the user of today regarding the accuracy in printing devices.
After positioning the C-shaped optical encoder device on the PCB, the C-shaped optical encoder device must remain permanently attached to the PCB. This is usually done via one of the following three possibilities: using hot-riveted studs, using screws, and/or using snap catch devices. FIG. 4A to FIG. 4C show cross-sections of these three possible permanent attachments. All three figures show cross-sections of the C-shaped optical encoder device 201 as well as of the PCB. In FIG. 4A, the C-shaped optical encoder device 201 has a mounting stud 401 which is inserted in and projected through a hole in the PCB. The projecting part of the mounting stud 401 was hot-riveted such that the mounting stud 401 ends in a button like manner which has a larger diameter than the hole in the PCB. In FIG. 4B, the C-shaped optical encoder device 201 comprises housing brackets 402. Head screws 403 are passed through the housing brackets 402 as well as through corresponding holes in the PCB and are fixed by nuts 404 on the opposite side of the PCB with respect to the C-shaped optical encoder device 201. In FIG. 4C, the C-shaped optical encoder device 201 comprises snap catch devices 405 which are passed through suitable holes in the PCB. However, these possible permanent attachments have disadvantages in that they involve large-scale processing methods and/or additional housing material.